A high court in the capital Juba handed Gatdet the death sentence as well as a combined 21 years in jail for incitement and conspiracy against President Salva Kiir’s government.
Monyluak Alor Kuol, Gatdet’s former lawyer who resigned last month in protest against the case’s handling, said the sentencing violated a ceasefire signed in December, which called for the release of all prisoners and detainees.
“I am disappointed. Such trials are not supposed to take place at this time,” he said.
South Sudan has been ravaged by war since December 2013, sparked by a fallout between President Kiir and his then deputy Machar.
Kiir accused Machar of plotting a coup against his rule, an allegation the latter denied. Machar however went on to mobilize a rebel force to fight the government.
The war has killed tens of thousands and displaced millions others, prompting the UN early last year to rank South Sudan as Africa’s biggest refugee crisis.
A peace deal signed in 2015 saw Machar return to Juba in March 2016 to take up the position of first vice president under Kiir. The deal did not last long, as forces loyal to the two leaders clashed in July, forcing Machar to flee the capital.
The rebel leader is currently in South Africa where he went to seek medical treatment following those clashes, but has not been able to return to his country.
A ceasefire deal was signed in December 2017 in a bid to revive the 2015 peace agreement. Even this has been hit by violations from both factions, prompting the international community, the UN and regional leaders to approve a raft of sanctions targeting offenders.